1. Field of the Invention
This invention relates to an automatic shoe clearance adjusting apparatus automatically adjusting a shoe-to-drum clearance existing between a brake drum and a brake shoe braking the brake drum by a frictional engagement therebetween. More particularly, this invention relates to an automatic shoe clearance adjusting apparatus with an automatic shoe clearance adjustment disabling mechanism suspending the automatic shoe clearance adjusting operation when the temperature of the drum brake exceeds a predetermined degree.
2. Description of Related Art
An example of a drum brake employing this type of automatic shoe clearance adjusting apparatus is fully described in PCT published application number WO 01/01008. An example of a construction of the automatic shoe clearance adjusting apparatus is explained with reference to FIGS. 6 and 7.
An automatic shoe clearance adjusting apparatus 22 is comprised of a strut 23, an automatic shoe clearance adjusting mechanism and an automatic shoe clearance adjustment disabling mechanism. The automatic shoe clearance adjusting mechanism automatically adjusts the shoe-to-drum clearance depending on the wear of the lining of the brake shoe (not shown in figures) and the automatic shoe clearance adjustment disabling mechanism suspends the automatic shoe clearance adjusting operation when the temperature of the drum brake exceeds a predetermined degree.
Components of the automatic shoe clearance adjusting apparatus 22 will be explained with reference to FIGS. 6 and 7, wherein terms “upper” and “lower” and terms “left” and “right” are given their ordinary meaning as suggested in FIG. 6.
The strut 23 is composed of a rotatable nut member 24, a fitting member 25 fitting into a left side portion of the nut member 24 to support the nut member 24 allowing relative rotation of the nut member 24 while non-rotatably engaging with a left brake shoe (not shown in the figures), and a bolt member 26 screwed into a right side portion of the nut member 24 to support the nut member 24 while non-rotatably engaging with a right brake shoe (not shown in the figures).
The nut member 24 is cylindrically shaped and structured such that a left side portion of the periphery on the nut member 24 is stepped to form a smaller diameter periphery 24e; a toothed ring with a ratchet wheel 24b force-fits onto a longitudinal left side of the small diameter periphery 24e; and an internal thread 24c is formed in a through hole 24d from a right side thereof.
The Y-shaped fitting member 25 is arranged such that a flat, plated section 25c at the left side thereof has an engagement groove to be engaged with the left brake shoe; and a stem section 25b at the right side thereof slidably fits into the through hole 24d of the nut member 24 from the left side.
The Y-shaped bolt member 26 is also arranged such that a flat, plated section 26b at the right side thereof has an engagement groove to be engaged with the right brake shoe; and an external thread 26a formed on the peripheral surface of a stem section 26c at the left side thereof is screwed into the internal thread 24c of the nut member 24 from the right side.
A driving member 27 is jointed to the strut 23 so as to implement the automatic shoe clearance adjusting mechanism and the automatic shoe clearance adjustment disabling mechanism.
The driving member 27 is designed such that a pawl element 29 made of bi-metal is fixed to an arcuate leaf spring element 28 made from a plate spring at an intermediate portion of the leaf spring element 28 between both ends 28a, 28c by a rivet 30.
The driving member 27 is jointed to the strut 23 by means of a bracket 31 and a sleeve 32 both installed on the strut 23. The bracket 31 having a proximal section 31a is securely attached to the fitting member 25 so that the proximal section 31a is being abutted against a stepped section 25a of the fitting member 25. The sleeve 32 is relatively rotatably fitted onto the small diameter periphery 24e and abuts against a stepped section 24a of the nut member 24 to prevent a rightward movement of the sleeve 32.
Thereafter, the toothed ring with the ratchet wheel 24b force-fits onto the small diameter periphery 24e of the nut member 24.
While the arcuate leaf spring element 28 is developing a resilient force acting to move the nut member 24 and the fitting member 25 apart from each other, the ends 28a, 28c thereof are hooked on the bracket 31 and the sleeve 32 respectively.
At that time, the tip of the pawl element 29 resiliently contacts to make a ratchet engagement with the ratchet wheel 24b of the nut member 24.
Operation of the automatic shoe clearance adjusting mechanism with the above configuration will be explained next.
Upon activating the service brake, a pair of brake shoes (not shown in the figures) move away from each other, an axial force acting on the strut 23 by a spring force of a shoe return spring located outside of the figures is released; the nut member 24 and the fitting member 25 are urged to move away from each other due to the resilient force developed by the arcuate leaf spring element 28 of the driving member 27; the nut member 24 moves rightward with the bolt member 26 and the fitting member 25 moves leftward; and the tip of the pawl element 29 moves in a direction to rotate the ratchet wheel 24b of the nut member 24 due to the extension of the arcuate leaf spring element 28. As the lining wears off and a traveling amount of the tip of the pawl element 29 exceeds one pitch of the ratchet wheel 24b, the nut member 24 is rotated for one tooth pitch relative to the ratchet wheel 24b to unthread the nut member 24 out from the bolt member 26 to extend the overall length of the strut 23, thereby automatically adjusting the shoe-to-drum clearance.
Operation of the automatic shoe clearance adjustment disabling mechanism will be explained next.
For example, frequent use of service brake on a down slope raises a brake drum temperature to cause lateral expansion of the brake drum. This expansion automatically enlarges the shoe-to-drum clearance just like being enlarged due to a general lining wear. However, if there is no special treatment for the enlargement created due to the temperature expansion, normal automatic shoe clearance adjusting operation is conducted to provide same type of shoe clearance adjustment as provided for the enlargement created due to the lining wear. Yet, if the brake drum is cooled down to the ordinary temperature, the expansion does not exist thereafter and the enlarged shoe-to-drum clearance is reduced to the extent that is previously enlarged due to the expansion. According, the ordinary automatic shoe clearance adjusting operation conducted while the expansion being existed becomes over-adjustment. Such over-adjustment might be a cause for dragging of the brake.
On the other hand, considering the expansion problem during the service brake operation, if the shoe-to-drum clearance is initially set larger, the brake stroke under the ordinary temperature becomes longer and thereby causing the driver to experience an unusual or disconcerting brake feeling.
For the purpose of resolving the above-identified problem, the pawl element 29 is made of bi-metal. Accordingly, as the brake drum temperature rises, the temperature of the pawl element 29 rises as well. The pawl element 29 is thermo-reflected, as shown in the two-dot chain line of FIG. 7. When the temperature of the pawl element 29 exceeds a predetermined degree, the bi-metal causes the pawl element 29 to move away from the ratchet wheel 24b, thereby suspending the automatic shoe clearance adjusting operation. This ultimately solves the problem of the shoe clearance over-adjustment.
The above-described conventional automatic shoe clearance adjusting apparatus has the following drawbacks.
While the shoe clearance adjustment disabling mechanism is operating, an axial force acting on the strut 23 is released and the bi-metal pawl element 29 is separated from the ratchet wheel 24b of the nut member 24. When this happens the nut member 24 becomes free without an external force acting thereon and this results in an irregular movement of the nut member 24, thereby possibly causing an adverse effect on the shoe-to-drum clearance.
Another adverse effect is that the driving member 27 requires more than minimum number of components. In the prior art driving member 27 comprises the arcuate leaf spring element 28, the bi-metal pawl element 29, and a joint means such as a rivet 30 so as to fix the two. More components are necessary if the bracket 31 and the sleeve 32 are provided for the purpose of fitting the driving member 27 on the strut 23. In addition, due to the excess number of components, forming the bracket 31 and the sleeve 32 requires more complex processing which in turn increases the manufacturing costs.
Accordingly, because of above-mentioned structure, assembling the components becomes complicated, thereby increasing the amount of the assembly task. Therefore, the manufacturing costs increases.